Thermal transfer printers generally print images using a line-type thermal head including heat generators arrayed in a line, specifically by applying heat to a thermal transfer sheet and a receiving material in overlap while moving them in the direction perpendicular to the longitudinal direction of the thermal head.
Thermal transfer sheets that are used for such image-printing include a heat resistant slipping layer, and the heat resistant slipping layer is formed on a surface of a base material that is to be in contact with a thermal head for the purpose of improving heat resistance and ensuring traveling stability by adding slip property. Recently, an increase in printing speed of printers has resulted in an increase in thermal energy applied by a thermal head. Therefore, the thermal transfer sheets are required to have better heat resistance and slip property.
Various methods of forming the above heat resistant slipping layer have been known.
For example, Patent Document 1 discloses a method of forming a heat-resistant layer of a high heat-resistant thermosetting resin. In this method, however, the heat resistance is improved, but the slip property for a thermal head is insufficiently improved. Further, this method involves use of a curing agent such as a cross-linking agent, and therefore, two-part coating liquid is needed. This poses problems such as pot life. In addition, since a plastic thin film, which can not undergo high-temperature treatment, is used as a base material, the coated resin needs to be thermally treated (aged) at low temperature for a long time in order to give a sufficiently-cured coating film. This complicates the production processes, and further poses problems in that defects such as occurrence of cockles and blocking are often generated.
Patent Document 2 discloses, as a heat resistant slipping layer of a noncurable resin, a heat resistant slipping layer excellent in printing stability and traveling stability. The heat resistant slipping layer is formed from a resin composition which includes a binder containing a mixture of specific amounts of a polyamideimide resin and a polyamideimide silicone resin, a polyvalent metal salt of alkyl phosphate, and a filler.
These heat resistant slipping layers achieve improved productivity and slip property, but still have problems in that a defect called “tailing” is caused due to the heat resistant slipping layer when a high-print density portion, which needs a high energy application, is printed, followed by printing of an image including a low-print density portion. Here, the term “tailing” means a phenomenon in which a component of the heat resistant slipping layer that has been fused by a high energy applied during printing of the high-gray-scale portion is drawn with thermal energy being stored, and then causes problems in color development during printing of a low-print density portion. The occurrence of such tailing, mainly caused by the heat resistant slipping layer, becomes more marked as the thermal energy applied at the time of printing increases along with an increase in printing speed.
Meanwhile, Patent Document 3 discloses a thermal transfer ribbon including a polyester film, a colored ink material layer disposed on one surface of the polyester film, and a polyamide resin layer that is disposed on the other surface thereof and has specific physical properties and essentially contains an isophorone residue and a C4-12 aliphatic dicarboxylic acid.
This polyamide resin layer can suppress the occurrence of tailing, but has poor friction stability to a high-gray-scale portion and insufficient heat-resistance because its slip property is improved only by copolymerization or modification of a polyfunctional silicone compound. Therefore, it has been required to develop thermal transfer sheets that have good heat resistance and slip property and can prevent the occurrence of tailing.    Patent Document 1: Japanese Kokai Publication Sho-61-14983    Patent Document 2: Japanese Kokai Publication 2001-334760    Patent Document 3: Japanese Kokai Publication Hei-7-314929